The invention may advantageously be applied, but is not limited to, wireless electronic systems capable of exchanging such signals with microwaves, millimeter and terahertz wavelengths.
The HDMI standard is a wired video data transmission standard. The data rates are very high. In order to obtain such a wireless transmission (W-HDMI), the use of a 60 GHz frequency is proposed with a very high data rate (between 3 and 6 Gb/s) and over distances from 3 to 10 meters between two transmitters/receivers for which the nature of the path of the waves between these two elements can be direct (LOS or Line-of-Sight) or indirect (NLOS or Non-Line-of-Sight) using the acronyms that are well known to those skilled in the art. An antenna or an antenna array must then be used whose radiation pattern in transmission and reception is steerable and a system is needed with a high wireless transmission gain (or “air link gain” according to a term well known to those skilled in the art).
There are then two possible alternatives for the implementation of this system. A first alternative aims to use a power amplifier with a high output power connected to an antenna or antenna array having a moderate gain. This then leads to a high power consumption. Another alternative aims to use a power amplifier with a moderate output power connected to an antenna or antenna array having a high gain. This then leads to a reduced power consumption of the system but the antenna or the antenna array generally requires additional external devices (for example a lens) in order to achieve a high gain.
With an antenna array, it is possible to obtain an electronic pointing of the array in one direction by varying the phase and the amplitude of each of the signals sent to and/or received from the antennas of the array. Indeed, depending on the various phase shifts, the direction of the radiation pattern of the antenna array can be adjusted. Moreover, in a given direction, a higher gain can be obtained than with a single omni-directional antenna.
For the elements of the antenna array, planar antennas or non-planar antennas may be used. The literature provides exemplary embodiments of antennas.
Thus, the publication entitled “High-Gain Yagi-Uda Antennas for Milimeter-Wave Switched-Beam Systems”, by Ramadan A. Alhalabi and Gabriel M. Rebeiz in IEEE TRANSACTIONS ON ANTENNA AND PROPAGATION, VOL. 57, NO. 11, NOVEMBER 2009, describes a high-efficiency power supply for an antenna known as a Yagi Uda antenna for millimeter wavelengths using a microstrip system. This antenna is constructed on either side of a teflon substrate which allows the passage from a symmetric transmission line (antenna) to an asymmetric transmission line (microstrip). A gain of 9-11 dB is thus obtained for frequencies in the range 22-26 GHz. When used in an array of two antennas, a gain of 11.5-13 dB is obtained for the frequencies 22-25 GHz. A high radiation efficiency is obtained.
The publication entitled “On-Chip Antennas for 60-GHz Radios in Silicon Technology” by Y. P. Zhang, M. Sun, and L. H. Guo in IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 52, NO. 7, JULY 2005, describes a compact and efficient antenna for 60 GHz radio waves. This antenna is fabricated on a silicon substrate with a low resistivity of 10 Ω·cm. Two types of antennas have been used, namely an antenna of the Yagi-Uda type and an antenna referred to as an inverted-F antenna. The results obtained are respectively the following: for the inverted-F antenna, insertion losses of 32 dB and a gain of −19 dBi at 61 GHz, and for the Yagi-Uda antenna, insertion losses of 6.75 dB and a gain of −12.5 dBi at 65 GHz (with dBi a unit well known to those skilled in the art representing in dB the gain of an antenna with respect to an isotropic aerial, in other words an antenna which is capable of radiating or of also receiving in every direction and for every polarization).
The publication entitled “60 GHz Antennas in HTCC and Glass Technology” by J. Lanteri, L. Dussopt, R. Pilard, D. Gloria, S. Yamamoto, A. Cathelin, H. Hezzeddine from EuCAP 2010, describes an antenna constructed on glass and connected to a ceramic module using the ‘flip-chip’ technique. An antenna array comprising two antennas such as described hereinabove has also been fabricated. The results obtained are the following: for the single antenna, insertion losses less than 10 dB and a gain of 6-7 dBi over a bandwidth at −10 dB of 7 GHz and, for the antenna array, a gain of 7-8 dBi over a bandwidth at −10 dB of 3 GHz.
When an antenna array using a single type of antenna is employed, for example antennas of the planar type, the radiation pattern of the array can be degraded for large pointing angles with respect to the normal to the plane formed by the antenna array. This is notably the case when the electronically pointed directions make a large angle θ(theta) in the plane of the electric field with the normal to the plane of the antenna, in the radiating direction.
FIGS. 1 to 3 illustrate this problem in the particular case of planar antenna arrays. FIG. 1 shows an antenna array RE comprising 4 planar antennas E1, E2, E3, E4 having the same orientation and the same radiation pattern. The distance between the barycentres of E1 and E3 is equal to the distance between the barycentres of E2 and E4 and the distance between the barycentres of E1 and E2 is equal to the distance between the barycentres of E3 and E4. Accordingly, the antenna array is one in which the barycentres of the antennas are mutually equidistant, and typically separated by λ0/2, λ0 being the wavelength in air of the signal to be transmitted or received.
The planar antennas E1, E2, E3, E4 are identical and a more detailed representation is shown at the bottom of FIG. 1. In fact, a planar antenna is for example formed from a substrate SB represented by the large parallelepiped onto which a conducting surface SC, represented by the small rectangle on the surface, is bonded or connected.
FIGS. 2 and 3 show radiation patterns as a function of the orientation of the electromagnetic waves to the normal to the planar antennas in the plane of the electric field, for the antenna array according to FIG. 1. For the sake of clarity, the 7 curves shown have been distributed between FIG. 2 (C1, C2, C3, C4, C5) and FIG. 3 (C6, C7).
The curve C1 represents the radiation pattern of one of the elements E1, E2, E3 or E4 as a function of the orientation of the electromagnetic waves to the normal from the elements E1, E2, E3 or E4.
The curve C2 represents the theoretical radiation pattern for the antenna array as a function of the orientation of the electromagnetic waves in the plane of the electric field. This pattern is determined by adding to the curve C1 the value: “10 log (N)” for N elements, in other words 10 log (4) with 4 elements E1 . . . E4. The notation log represents the logarithmic function in base 10.
Each of the curves C3, C4, C5, C6 and C7 illustrates, for a pointing direction making an angle θ (theta) with the normal to the antenna array RE in the plane of the electric field, the radiation pattern as a function of the orientation of the electromagnetic waves. The pointing direction is obtained electronically by applying various phase shifts to each of the signals from the elements E1 . . . E4.
The curve C3 corresponds to the case where no phase shift is applied to the antenna array. In this case, the maximum directivity of the radiation pattern is aligned with the direction normal to the planar antennas. The pointing direction makes an angle θ(theta) equal to 0 with the normal to the antenna array, in other words the pointing direction is in the same direction as the normal to the antenna array, this direction is also known as “azimuth”.
The curve C4 corresponds to the pointing direction making an angle θ (theta) equal to +35° in the plane of the electric field with the normal to the antenna array.
The curve C5 corresponds to the pointing direction making an angle θ(theta) equal to +70° in the plane of the electric field with the normal to the antenna array.
The curve C6 corresponds to the pointing direction making an angle θ(theta) equal to +80° in the plane of the electric field with the normal to the antenna array.
The curve C7 corresponds to the pointing direction making an angle θ(theta) equal to +90° in the plane of the electric field with the normal to the antenna array.
As can be seen, the pattern represented by the curve C3 comprises two side lobes for the orientations “+50°” and “−50°”. These are substantially reduced with respect to the main lobe) (0°).
The pattern represented by the curve C4 comprises a main lobe (+35°) and three side lobes at around the orientations “−10°”, “−45°” and “−85°”. These are also relatively substantially reduced.
The pattern represented by the curve C5 comprises a main lobe (+70°) and three side lobes around the orientations “+10°”, “−20°” and “−70°”. As can be seen, the side lobe along the orientation “−70°” has almost the same gain as the main lobe.
The pattern represented by the curve C6 comprises a main lobe (70°) with three side lobes at around the orientations “15°”, “−15°” and “−70°”. The side lobe along the orientation “−70°” has a gain equal to the main lobe. Moreover, the main lobe is not in the pointing direction but along an orientation making a smaller angle (+70°).
The pattern represented by the curve C7 comprises a main lobe (+70°) and three side lobes around the orientations “+10°”, “−20°” and “−70°”. The side lobe along the orientation “−70°” also has a gain equal to the main lobe. Moreover, the main lobe is not in the pointing direction θ(theta) equal to +90° but in a direction making a smaller angle (+70°).
The following are thus observed for electronically pointed directions making large angles θ(theta) with the normal:
a superposition of the main lobes for pointing directions making angles θ(theta) greater than 70°,
a degradation of the main lobe for pointing angles θ(theta) greater than 45°,
a generation of side lobes with a gain as high as the main lobes for pointing angles θ(theta) greater than 45°.
Several problems can then result: degradation of the aerial transmission gain in the lateral directions, problems of synchronization between the transmitter and the receiver, direction of the transmission not well defined, generation of several paths (due to the side lobes) and appearance of interference effects.
Several conventional techniques exist for reducing (or “tapering” according to a term well known to those skilled in the art), the side lobes in the case of an antenna array.
One of the known techniques (“amplitude tapering” according to a term well known to those skilled in the art) consists in adjusting the amplitude of the signals from each of the antennas. This solution can thus be implemented by an electronic management system. However, it is difficult to control the relative amplitude of each antenna for the numerous orientations of the waves to be transmitted and/or received.
Another solution consists in adjusting the phase of the signals from each of the antennas (“phase tapering” according to a term well known to those skilled in the art). This solution can also be implemented by an electronic management system, but it is very complex to control and may even be incompatible with the pointing techniques using the phase.
Another technique consists in spacing the various antenna elements by non-uniform distances, but the antenna array obtained could then get very large.